Radio location techniques are widely used in many mobile transmitter systems. Among these location techniques are a) Time of Arrival (TOA), b) Time Difference of Arrival (TDOA), and c) Angle of Arrival (AOA) methods. Each of these techniques requires either the measurement or estimation of one or more parameters of the communication channel utilized by the mobile transmitter.
To estimate the location of a mobile transmitter in a code division multiple access (CDMA) communication system, estimates of channel parameters such as delay, amplitude, and possibly phase must be obtained for multiple signals. In general, these parameter estimates will be obtained by correlating received signals with one or more "perfect" reference signals.
Correlation with a reference waveform can also be interpreted as despreading followed by coherent averaging. Up to a point, the longer the correlation (coherent averaging length), the more despreading gain is obtained. Effects from noise and interference on these channel estimates, which are normally severe, can be made arbitrarily small so long as the despreading gain can be increased. In practice, the maximum despreading gain is limited by the non-stationarity of the communication channel. If the communication channel lacks sufficient stationarity, a longer averaging window will ultimately result in a decrease in despreading gain below the maximum achievable level. Thus, it is desirable to determine the optimal coherent averaging length and the corresponding maximum despreading gain for the radio channel conditions achievable at any given observation.
Higher gain is advantageous particularly in mobile transmitter location applications. This is because it may be necessary for an adjacent sector or remote sector to detect the mobile transmitter to accurately fix its position. In a communication system, such as a CDMA communication system where the mobile transmitter is power controlled, when the mobile transmitter is located near to a serving sector, it may communicate with relatively low power. As a result, detection of the mobile transmitter's signal at adjacent or remote sectors is difficult. Where detection is required, such as when determining the mobile transmitter's location, it has been proposed to have the mobile transmitter "power-up" to a level sufficient to permit detection at the adjacent or remote serving sector. Such a strategy, however, defeats the benefits of providing power control.
Therefore, a need exists for a method and apparatus for channel parameter estimation having high gain under various communication channel conditions.